Gantry cranes have been used for many years for lifting and handling loads such as truck trailers, cargo containers, boats and the like. The cranes normally have a gantry structure that spans over the load(s). For example, in intermodal applications, it may span over two adjacent railroad cars or a truck trailer adjacent a railroad car. The gantry structure normally is comprised of a pair of side support frames, which are spaced by trolley beams. Each side support frame is generally comprised of two vertical legs connected at their bottom ends by a lower side beam and connected at their top ends by an upper side beam. Each side support frame defines a vertical plane.
Conventionally, a lift frame is suspended by flexible cables from the trolley beams. Spreaders, having a pair of descending arms, are located at or near each end of the lift frame. Each arm has a pivot shoe, or finger, that can engage a bottom side of a load, such as a truck trailer so that the load is engaged proximate its four corners. In some crane designs, the spreader arms can rotate to an inoperable position so that optional, specialized twist locks, can be provided on the spreader to engage a load, such as a cargo container, from the top of the container. Such a design allows intermodal operation of the gantry crane. In one mode, the crane moves loads using the spreader arms. In the other mode, the crane moves cargo containers using the twist locks.
Generally, an operator platform or an enclosed cab is provided on the gantry structure. From the platform or cab, an operator controls all of the movements of the gantry crane, i.e., the lift frame, spreader arms and the mobility of the gantry crane itself. Conventionally, the operator platform or cab is mounted either on the lower side beam of a side support frame or to one of the vertical legs.
One problem encountered by operators of gantry cranes is limited vision from the operator cab. With the operator cab in a fixed position, the operator's line of sight is also fixed. In intermodal operation, for example, the operator is unable to vary his line of sight regardless of the mode of operation or the type of load being lifted.
For instance, when a cargo container must be engaged at its top by the twist locks on the lift frame, it can be difficult for the operator, while sitting in the cab a few feet from ground-level, to properly align the twist locks with the top of the container. As a result, the operator requires more time to engage and move the container, therefore, lowering operation efficiency. In addition, the operator can damage the top of the container while attempting to properly align the twist locks with the container. Without seeing the top of the container, the operator must estimate the correct position of the twist locks and may accidently stab the container with the twist locks.
One proposed structure for allowing the operator to change his line of sight is disclosed in U.S. Pat. No. 4,877,365. Disclosed therein is a crane having a lift frame suspended from a gantry structure. The crane is provided with a first operator cab positioned near ground-level below a lower side beam of one side frame. The crane has a second operator cab positioned at an elevated position within the gantry structure. Thus, the operator can vary his line of sight by controlling the crane either from a position at ground-level or from an elevated position. This design is inconvenient, however, because the operator must leave one cab and then travel to the other cab to improve his view. Such a design also requires two sets of controls and associated wiring and instrumentation which increases costs. Finally, the design only gives the operator two vantage points, rather than allowing the operator to choose from a number of lines of sight.
The operator's limited vision from an operator cab positioned near ground-level can also cause problems when maneuvering the crane through a warehouse or railroad yard. With the cab near ground-level, it is difficult for the operator to steer the crane through stacks of containers or randomly-spaced truck trailers. As a result, the gantry crane can accidentally collide with these objects.
Another problem encountered by operators is the inability to vary horizontal lines of sight. In certain instances, an operator may have difficulty in aligning front and rear ends of the lift frame with a load to be lifted. Thus, it would be desirable for an operator to obtain a more direct view of the front and rear ends of the lift frame and load to be lifted. As discussed, however, the operator cab is normally fixed in one position on the gantry crane. Even if the operator cab was mounted for vertical movement, allowing varying vertical lines of sight, the operator would still be unable to vary horizontal lines of sight from the front end of the lift frame to the rear end of the lift frame.
Thus, there is a need for a gantry crane having an operator cab that allows an operator to vary his line of sight to efficiently engage a load either from its bottom or from its top, and also to efficiently engage a load at its front and rear ends. The present invention is provided to solve these and other problems.